You'd expect that but, no. That's why you cannot load glibc-linked binaries in a Musl distro. Edit: that's why the hacks like the original post is needed, as well.
The ABI is strongly dependent on explicit libc implementation in current Linux systems. There is no libc independent ABI on Linux.
Sorry, can you be more specific. I do not understand what the problem is. If Musl does not implement support for the ABI, this would be a musl problem?
There is no libc independent ABI. ABI doesn't purely mean just calling conventions.
When you compile libc, you also get a binary loader ld-linux.so with it. They are not two independent components of a system.
Basically all .so files compiled with glibc require the ld-linux.so that's also generated by that glibc (or a later version, if they didn't break the binary compatibility).
There are a lot of stuff that's executed by ld-linux.so and glibc that are not explicitly documented but they are absolutely necessary for your program to start and correctly initialize things like global variables or signal handling or loading other dynamic libraries. Some of that functionality sits in ld-linux.so and some of that in glibc. They have circular dependencies to each other. glibc expects ld-linux.so to put things in certain order but ld-linux.so also must load glibc first to have access to certain APIs. They are not part of System V ABI. They are not documented.
Musl maybe can implement this but it is simply reverse engineering what glibc did and then playing a game of cat and mouse. There is no independent ABI standard.
Sorry, again this too vague for me. What is the exact problem with atomics and thread_local in C that would make the ABI dependent on a specific version of glibc? I know the ABI is not just calling convention and e.g. for atomics may involve calling a function from libatomic. But from my understanding, this is all part of a standardized ABI that does not change and can be provided by different implementations.
Then, what is the exact reason a library compiled against glibc must be loaded by a specific ld-linux? I could see that this is true for C++ perhaps, or when you use very special features, but I do not see this for C.
I often compiled programs against one version of glibc and run it against a different version, so I know there is not a tight coupling. So please be specific in explaining in what scenarios this would break.
> Then, what is the exact reason a library compiled against glibc must be loaded by a specific ld-linux? I could see that this is true for C++ perhaps, or when you use very special features, but I do not see this for C.
You still have functionality like `dlopen` with C or `pthreads` with C. ld-linux.so is the thing that prepares the stack frame, or the segment pointers (FS_BASE on x86_64).
When you have your main loaded __libc_start_main_impl is called from glibc and if you disassemble it you'll see this line:
call 0x7ffff7c28790 <_dl_audit_preinit@plt>
Guess where _dl_audit_preinit lives?
(gdb) info symbol _dl_audit_preinit
_dl_audit_preinit in section .text of /lib64/ld-linux-x86-64.so.2
> I often compiled programs against one version of glibc and run it against a different version, so I know there is not a tight coupling. So please be specific in explaining in what scenarios this would break.
Thanks, I need to dig into this more. But I still not convinced there is a real issue here. Of course different infrastructure needs to exist for different feature. What exactly is the underlying issue with thread_local in your specific scenario? It seems Musl does not setup the the infrastructure needed for ABI-compliant thread-locaL data access?
THERE IS NO STANDARD, AGREED ON, RFC'd or EVEN BEHAVIORALLY DOCUMENTED ABI FOR MUSL TO FOLLOW!
It's not Musl's fault that Glibc has an undocumented, badly designed, tightly coupled implementation. It's not Musl's fault when they decide to not follow Glibc's design which they have to reverse engineer and rewrite from scratch. Then Glibc can break it in a future version anyway. They give 0 guarantees and have a track record of breaking things.
Sorry but I feel like you're trying to excuse Glibc out of their terrible design. Glibc didn't ask anybody for standardization. Glibc didn't document their behavior. Musl doesn't need to follow any undocumented behavior. Musl has its own implementation of thread_local variable and it works only for Musl dynamic binaries (which you cannot load with Glibc's ld-linux.so either). You need to tell your compiler to generate code that Musl prefers not Glibc. So effectively we have two ABIs.
There is no standard and any design that tightly couples the system binary loader to the C standard library is just extremely terrible design. Both Musl and Glibc do it. Both of them are terrible. Glibc was first and it set the horrible, binary hostile behavior "have no standards, trust Glibc, and recompile your programs again" as the "standard".
The entire Linux userspace depends on Glibc's terrible design. Due to Hyrum's Law, the exact needs of the complex and emergent behavior can only be surfaced via making a better designed, completely independent system loader and a completely independent libc and then fixing the entire tens of thousands of userspace libraries in the upcoming decades. It is just Sisyphean work to fix Linux userspace.
There is no simple "let libc-X obey the standard S" solution. There is no standard. Linux Standard Base project tried to set a standard. It failed.
I'll won't further discuss this with you. Maybe you have good intentions, maybe you're playing dumb. I'm not sure anymore. If you're the former, I'm not going to deliver every single bit of information. I provided enough resources and you can learn.
